It probably depends on where he will be locking it up, is it visible from the road, how good is his bike, versus what other bikes are there and if they are locked up with cheap rubbish. The couple of bikes that have been nicked from my children's school shed have usually not been locked up, or left overnight. But at another school nearby the bike shed is next to the road, not overlooked and loads have gone missing from it.
I'd go with the D-lock left at school option.
Its also worth spending 5 mins showing him how to lock it up... came back one day to find my middle child (aged 11) had locked up her bike using the handlebars!!!! - luckily with a small D-lock but not very secure if you had an allen key. And alot of the bikes in the bike shed are often just locked up by their quick release wheel!